1. Field of the Invention
The present invention relates to a speed control method for a Switched Reluctance Motor(SR motor), and more particularly to a speed control for a SR motor when employing a load instance response of which is not large, that is a load having a large mechanical inertia.
2. Description of the Conventional Art
Generally, a pulse width modulation control and a current control are used for an SR motor speed control method. Particularly, the PWM control method controls the operation speed of the SR motor by applying a pulse width modulated signal to a switching transistor on the basis of a pulse corresponding to an output value of a detection sensor which detects a speed, while the current control method prevents an overcurrent from flowing to motor coils by setting a hysteresis band width of a current in a switching interval wherein the switching transistor is switched and switching the switching transistor so that the current applied to the motor coils exists in the hysteresis band width which has been set.
FIG. 1 is a sectional diagram illustrating a three-phase SR motor in general. As shown therein, 10 is a rotor, 20 is a stator, and La,Lb,Lc are coils winding the stator 20.
FIG. 2 is a circuit diagram illustrating a control circuit for controlling a speed of the SR motor in FIG. 1. As shown therein, the control circuit includes an inverter 21 consisting of six switching transistors Q1-Q6, freewheel diodes D1-D6 and a direct current DC condenser C connected with a power supply in parallel, a detection sensor 22 detecting a rotation of the rotor of the motor and outputting a signal in accordance with the detection, a speed detecting unit 23 determining a location of the rotor from the signal outputted from the detection sensor 22 and thereby outputting a detection pulse signal ps, and a speed control unit 24 outputting a plurality of switching signals cs1-cs6 to gates of switching transistors Q1-Q6, respectively, of the inverter 21, the three upper switching transistors Q1-Q3 being serially connected with the three lower switching transistors Q4-Q6 through the coils La,Lb,Lc.
FIG. 3 is a diagram illustrating waveforms of the switching signals cs1,cs4 applied to the gates of the switching transistors, respectively, and a current ia accordingly applied to the A-phase coil La when the three-phase SR motor is controlled in the PWM voltage control method. The speed detecting unit 23 detects a present location of the rotor 10 from the signal supplied from the detection sensor 22 and accordingly outputs detection pulse signals ps1,ps2 to the speed control unit 24 whenever the rotor 10 rotates by predetermined degrees, for example, 60.degree.. When receiving the detection pulse signal ps1, the speed control unit 24 outputs the fourth switching signal cs4 at a high level to the switching transistor Q4 and outputs the first switching signal cs1 repeating the high and low states, that is the pulse width modulated signal, to the first switching transistor Q1. The fourth switching transistor Q4 maintains an on state in accordance with the fourth switching signal cs4, while the first switching transistor Q1 alternately becomes on and off in accordance with the first switching signal cs1. Here, as shown in FIG. 3, a pattern of the current ia flowing to the A-phase coil La has a saw-tooth waveform a size of which gradually increases. When the rotor continuously rotates, that is taking an example of the three-phase, when the rotor rotates by 60.degree. after the second pulse signal ps1 is generated, the speed detecting unit 23 outputs the second detection pulse signal ps2 to the speed control unit 24. Accordingly, the speed control unit 24 outputs the first and fourth switching signals cs1,cs4 at the low state, whereby the first and fourth switching transistors Q1,Q4 are turned off and the current is flowing to the A-phase coil La starts decreasing and eventually becomes zero. The speed control unit 24 outputs the second and fifth signals cs2,cs5 at the high state, whereby the current flows to the B-phase coil Lb (not shown).
Here, when increasing a pulse duty of the first switching signal cs1, it is possible to increase the speed of the motor. Then, since an average time for which the first switching transistor Q1 is turned on while the fourth switching transistor Q4 is turned on increases, an increase interval of the A-phase current ia lengthens and a decrease interval thereof shortens so that the speed of the motor increases.
While, when the load increases, the speed of the motor decreases in proportion to the increased volume of the load, and accordingly an interval T between the first detection pulse signal ps1 and the second detection pulse signal ps2 lengthens. That is, the interval T which is a mechanical angle becomes larger than 60.degree.. In this case, the time for which the fourth switching transistor Q4 is turned on lengthens and the switching time for the first switching transistor Q1 lengthens, whereby the A-phase current ia increases. Further, when the speed of the motor considerably decreases due to radical increase in the load, the interval T abruptly lengthens, which means the switching interval T of the first switching transistor Q1 lengthens, so that the size of the A-phase current ia radically increases, thereby possibly exceeding a rated current. Here, the rated current means a current value above a level by which the motor and the inventor can be damaged. In this case, to prevent the break-down of the motor due to the excessive current, it is required to provide a separate current protecting circuit to cut off the excess current flowing to the system. To provide such a current protecting circuit a complicated circuit is needed and accordingly the manufacturing cost of the system increases.
To prevent the excess current, a control method is applied, which previously sets a predetermined current hysteresis band width and controls the current value not to exist out of the hysteresis band width, such method being called a current control.
FIG. 4 is a diagram illustrating waveforms of the switching signals cs1,cs4 applied to the gates of the switching transistors, respectively, and a current ia accordingly applied to the A-phase coil La when the three-phase SR motor is controlled in the current control.
When the first detection pulse signal ps1 is generated, the speed control unit 24 outputs the first and fourth switching signals cs1,cs4 at a high state and accordingly the A-phase current ia increases. When the value of the A-current ia which has been increasing exceeds the current hysteresis band, the speed control unit 24 turns off the first switching transistor Q1 and thus the A-current ia starts decreasing. When the value of the A-current ia which has been decreasing becomes lower than the current hysteresis band width, the speed control unit 24 turns on the first switching transistor. Such a process is continuously repeated until the second detection pulse signal ps2 is generated. Thus, the A-phase current ia does not exceed the hysteresis band width, thereby preventing the overcurrent from flowing to the A-phase coil La.
In the PWM control system and the current control system, a switching frequency is a radio frequency, which is about 15-20 kHz, thereby preventing noises produced when the switching transistor is switched. However, to perform the high-speed switching an expensive power switching device such as an integrated gate bipolar transistor (IGBT) or a field effect transistor (FET) is required and also a switching loss due to the high-speed switching is unavoidable.